This invention relates to expanded cellular products prepared from polyethylene and related polymeric substances.
Polyethylene foams prepared from low density polyethylene resins (xe2x80x9cLDPExe2x80x9d resins) have been widely accepted for industrial uses. Typically, these foams have light weight and a high degree of uniform enclosed fine cells. LDPE foams can be produced with densities in the range of from about 1 to 30 pcf (16 to 480 kg/cubic meter). Polyethylene foams generally have low water vapor transmission properties and are resistant to mechanical and chemical deterioration. Polyethylene foams are particularly suitable for use in thermal insulation, flotation, cushioning, and packaging. LDPE resins exhibit good melt strength desirable for foaming by conventional methods.
LDPE is made by the so-called xe2x80x9chigh pressurexe2x80x9d process by polymerization of ethylene in the presence a suitable catalyst. LDPE typically has a relatively low density of from about 0.91 g/cc to less than about 0.94 g/cc, typically about 0.92 g/cc. The good melt strength of LDPE is usually attributed to the long-chain and short-chain branches that are distributed along and extend from the polymer backbone. These branches make it more difficult for the individual molecules to slide over each other, which increases the resistance of the molten polymer to stretching during elongation. Increased resistance to stretching is sometimes referred to as xe2x80x9cextensional viscosityxe2x80x9d and is indicative of the melt strength of the resin and of the ability of a polymer to produce stable, high quality foams of low density. The cell walls formed by nucleation of bubbles during the foaming process offer sufficient resistance to expansion and do not become thin and collapse.
Efforts have been made to produce foams from so-called xe2x80x9clinearxe2x80x9d polyethylene resins. Generally speaking, linear resins have poor melt strength and are considered unsuitable for making lower density foams. Whereas LDPE is relatively highly branched with widely spaced chains and can be compared to dead tree branches piled together, linear resins are characterized by long, straight chains with less branching and so the molecules are more closely aligned in the manner of carefully folded rope.
Polyethylene resins become more difficult to foam as density, and linearity, increase. For example, unlike LDPE, high density polyethylene (xe2x80x9cHDPE)xe2x80x9d is produced in a low pressure process and has a relatively high density of from about 0.94 g/cc to 0.96 g/cc. HDPE molecules are among the most linear of the polyethylenes and have a small, controlled number of short-chain branches and normally have essentially no long chain branching. HDPE usually has a higher degree of crystallinity than LDPE and is physically a stiffer, stronger substance than LDPE. HDPE is typically about 70% crystalline at room temperature while LDPE may be as low as about 30 to 45% crystalline. HDPE has a higher flexural modulus and increased thermal stability as compared to LDPE as indicated in part by its higher melting point, and these properties would be useful in expanded cellular products. However, the individual molecules in an HDPE melt can slide over each other easily, and thus HDPE generally exhibits poor melt strength and low extensional viscosity. When HDPE polymers are used in foaming processes, these drawbacks frequently result in a large faction of open cells, foam collapse, and process instability. The cell walls of the HDPE foam normally do not have sufficient resistance to expansion and become thin and collapse.
Mixing branched and linear resins has been attempted in the production of extruded foams and other products, including films. Unlike films, which can be produced from such a mixture, foams require a uniform crystallization of the polymer molecules upon cooling of the expanded resin. The different crystallization characteristics of linear and branched polyethylene resins in physical admixture typically produce foams having large voids.
There are a large number of variables that impact whether a given resin is useful for foam production, including melt index, extensional viscosity, the presence of a cross-linking agent, and other parameters. Good quality foams have been made in which linear polyethylenes are a component under certain circumstances. For example, a cross-linking agent can be activated after extrusion to assist the foam is holding its extruded shape.
Many methods in the art employ cross-linked polyethylenes in the foaming process. Cross-linking enables the extruded foam to retain its shape. For example, HDPE resin can be extruded to the desired shape, cross-linked, and then expanded, normally by a chemical blowing agent that is activated after extrusion and cross-linking in a process called the xe2x80x9ctwo stage process.xe2x80x9d The resin is extruded prior to cross-linking because the shape of the product is fixed after cross-linking and cross-linking strengthens the resin to withstand expansion by a blowing agent. The two-stage process is in contrast to single stage extrusion foaming, in which a physical blowing agent, including, for example, a volatile organic compound, is mixed under pressure with a molten LDPE resin and then the mixture is extruded into a zone of lower pressure so that the blowing agent expands upon extrusion to produce the foam.
Cross-linking can lead to gelation of the ethylene polymers, which are undesirable localized concentrations of polymer more highly cross-linked than the surrounding areas, and can decrease the melt extensibility of the polymers. As a result, foams made with cross-linked HDPE generally have relatively high density, which is undesirable in many applications.
Methods have also been proposed for increasing long chain branching in the absence of cross-linking, typically by application of radiation. These methods can require steps that increase the complexity of processing the polymer. For example, U.S. Pat. No.5,508,319 describes a process for improving strain hardening elongational viscosity in linear polyethylene polymers such as HDPE and LLDPE in the absence of cross-linking. The polyethylene is irradiated with high energy ionizing radiation at a radiation absorbed dose of 2.0 megarads or less in an environment having an oxygen content of less than 15% by volume. The irradiated polyethylene is maintained in the environment for a period of time and is then treated to deactivate the free radicals present in the irradiated material. The resulting ethylene polymer is said to have a substantial amount of long chain branches without cross-linking and to exhibit improved melt strength and elongational viscosity.
U.S. Pat. No. 4,598,128 describes a method for making a polyethylene composition having enhanced temperature sensitivity and high low-shear viscosity. The composition is a blend of a linear polyethylene and a long chain Y-branched polyethylene. The Y-branched polyethylene is prepared by irradiating a polymer comprising molecules having at least one vinyl end group per molecule under non-gelling conditions in the absence of oxygen. It is disclosed that the vinyl end group can be created by heating an ethylene polymer under non-gelling, non-oxidizing conditions. The irradiation process is purported not to cause cross-linking.
Mobil Oil Company has recently marketed a group of HDPE resins designated as the HFE-03X series that are said to have sufficient melt strength to produce stable foams. While not wishing to be bound by theory, the Mobil resin is believed to be a xe2x80x9creactorxe2x80x9d resin that is a linear resin, but is produced with some degree of branching during the polymerization process that is favorable for producing a foam. The Mobil HFE-03X series resins are among the highest melt strength high density polyethylene resins available and have among the highest extensional viscosities available. However, stable lower density foams comparable in density to foams that can be made from LDPE , are not believed to have been achieved with these resins.
It would be desirable to produce stable, closed cell polyethylene foams of the lowest possible density from linear polyethylene resins in the absence of the drawbacks and disadvantages of complex processing steps and special environments and to increase the available options for producing high quality foams.
The invention is based on the recognition that linear ethylenic polymer resins can be produced that are compatible with highly branched low density polyethylene resins and can be admixed therewith to produce a resin having uniform crystallization behavior necessary to produce stable closed cell foams of the lowest possible density. A single melting temperature region can be observed for an intimate admixture of linear and branched resins, as opposed to distinct melting regions, which is indicative of uniform crystallization behavior. Quality closed cell foams, having about 80% or more of the cells closed, can be produced at low density in the range of from about 0.7 to less than 8 pcf (11 to 128 kg/cubic meter), and typically in the range of from about 0.7 to less than 4 pcf (11 to 64 kg/cubic meter). Foams can be prepared in the absence of branched polyethylene having a density as low as 2 pcf (32 kg/cubic meter). Scrap materials can be recycled and used to prepare these foams.
Foams prepared from irradiated linear resins and irradiated linear resins blended with LDPE, in accordance with the invention, can exhibit higher flexural modulus, stiffness, and tear strength for a given density than do foams normally obtainable from LDPE alone at the same density. Foams from linear low density polyethylene (LLDPE) resin show more balanced tear resistance in the machine and cross directions than has previously been achieved. Temperature stability is normally improved.
Improvements in physical properties of the foam are somewhat proportional to the amount of conventional LDPE in the blend. For example, up to 50% improvement in flexural modulus, which is a measure of the stiffness of the foam, can be achieved with as much as 40% of conventional LDPE in a blend with LLDPE. However, it should be recognized that conventional LDPE can be used in the blend in greater or lesser amounts, as desired, to optimize particular properties depending on the available resins, economic considerations, the intended use of the foam, and other factors.
The linear ethylenic resins useful in the practice of the invention should have a starting melt index of at least about 0.3 to 1 and are slightly irradiated in the absence of detectable cross-linking and in the absence special processing conditions. While not wishing to be bound by theory, it is believed that irradiation increases the branching in the linear resins so that when these resins are mixed with branched low density polyethylene (LDPE), then the branches become entangled and both temporary and permanent bonds are formed at the molecular level. Scrap shrink wrap film, which is normally a multilayer film and can include layers of irradiated resin, can be recycled for use in admixture with LDPE, up to about 60% by weight of the combined resins, to produce low density expanded cellular products at favorable economic conditions. Foams having enhanced or at least equivalent properties can be produced at lower cost.
Linear polymer structures suitable for irradiation in accordance with the invention include those polyethylenes normally considered in the art to be linear. It should be recognized that xe2x80x9clinearxe2x80x9d in the art and as used herein normally means that the ethylenic polymer may exhibit some degree of branching, usually introduced by comonomers or oligomers, although far less than xe2x80x9cbranchedxe2x80x9d low density polyethylene. For example, high pressure low density polyethylene, which is a highly branched structure, contains a relatively large number of both short and long chain branches, typically from about 10 to 30 per 1,000 carbon atoms.
The linear ethylenic polymers useful in the practice of the invention can be homopolymers or copolymers of ethylene with the alkyl derivatives of ethylene, which are also called alpha-olefins. These alpha-olefins usually have from about 3 to 20 carbon atoms in the chain and are added in relatively small amounts to modify structure, density, and crystallinity by introducing controlled branching and thereby disrupting the packing of the molecular chains. Additional linear ethylenic polymers suitable for foam production in accordance with the invention include copolymers and terpolymers of ethylene monomer or oligomer copolymerized or block polymerized with up to about 30% of generally bulky monomers. These monomers are usually selected from the group consisting of vinyl acetate; methyl methacrylate; maleic anhydride; acrylonitrile; alpha-olefins including propylene, butylene, and methyl pentene; isoprene; styrene; acrylic acid; and ionic salts of acrylic acid (ionomers). These various linear ethylenic polymers, copolymers, and terpolymers can be used alone or in admixture and as blends with conventional, highly branched low density polyethylene (LDPE).
The foams of the invention are prepared from linear resins that have been irradiated to reduce their melt index by at least about 20%. Extensional viscosity is increased by at least about 200% in the absence of cross-linking, complex processing steps, and special environments. Extensional viscosity can be increased above 250%, and typically by 350 to 450%. No reduction of oxygen is required. No free radical deactivation step is required. For example, high density polyethylene (HDPE) and linear low density polyethylene (LLDPE) resins treated in accordance with the invention can have extensional viscosity prior to expansion of from about 2xc3x97106 to 1xc3x97107 poise at 154xc2x0 C. for HDPE and at 140xc2x0 C. for LLDPE polymer at an extensional rate of 2 secxe2x88x921 using the Cogswell extensional viscosity technique mentioned in his textbook Polymer Melt Rheology by F. N. Cogswell, Woodhead Publishing Limited, Cambridge, England (1994). Linear resins having an extensional viscosity of from about 3.5xc3x97106 to 4.5xc3x97106 poise are somewhat more typical at the temperatures recited above.
The linear resins can be irradiated in air at ambient conditions of temperature and pressure and in the absence of detectable cross-linking, either before or after mixing with LDPE, if LDPE is used. The radiation dosage should be less than that threshold value that induces cross-linking. For example, the resin can be irradiated at 2.77 Mrads (megarads) at an ambient temperature of 72xc2x0 F. for about 40 to 45 seconds. However, it should be recognized that the radiation dosage is temperature and time dependent and so it is not possible to meaningfully set forth radiation dosages in the absence of a consideration of the conditions at which the radiation is applied. Normally, however, the radiation dosage will be from about 0.01 to about 4.0 Mrads at room temperature and pressure, for convenience, for a time sufficient to produce resins that can be expanded to a stable low density of less than about 8 pcf and in the absence of detectable cross-linking of the resin.
The resins treated in accordance with the invention are suitable for the wide variety of foaming processes known in the art, including, but not limited to, conventional extrusion foaming in which a blowing agent is mixed with molten resin under pressure and then extruded through a forming die into a zone of reduced pressure. A large fraction of uniform closed cells are formed, at least about 80% of the cells, and the foam is stable at low density. Other conventional methods for preparing polyethylene foams should also be useful, including, for example, two-stage expansion processes in which chemical agents are incorporated into the polyethylene resin that are capable of activation to generate a blowing agent in situ and thereby expand the resin to form a foam.
Thus, the invention provides compatibility between linear polyethylene polymers and highly branched low density polyethylene. A resin can be produced from an admixture of the two that exhibits a single melting range. The invention also provides a simple and cost effective method for making foams from various linear ethylenic resins and blends thereof and with low density polyethylene. Moreover, the expanded cellular products can be expected to exhibit improved flexural modulus, stiffness, tear resistance, tensile strength, temperature resistance, and melt strength at low densities. The foams of the invention have enhanced performance in a broad range of applications, including packaging, automotive, and recreational applications. Of primary benefit, scrap shrink film and the like can be used to economically produce very low density foams through recycling. Densities of from 0.7 to less than 4 pcf (11 to 64 kg/cubic meter), comparable to LDPE foams, can be achieved.